Integrated CZT(S,Se) Photovoltaic Device and Battery

ABSTRACT

An integrated kesterite (e.g., CZT(S,Se)) photovoltaic device and battery is provided. In one aspect, a method of forming an integrated photovoltaic device and battery includes: forming a photovoltaic device having a substrate, an electrically conductive layer, an absorber layer, a buffer layer, a transparent front contact, and a metal grid; removing the substrate and the electrically conductive layer from the photovoltaic device to expose a backside surface of the absorber layer; forming at least one back contact on the backside surface of the absorber layer; and integrating the photovoltaic device with a battery, wherein the integrating includes connecting i) a positive contact of the battery with the back contact on the backside surface of the absorber layer and ii) a negative contact of the battery with the metal grid on the transparent front contact. An integrated photovoltaic device and battery is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 15/281,870filed on Sep. 30, 2016, the contents of which are incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates to integrating photovoltaic devices withbatteries to provide recharging capabilities and more particularly, toan integrated kesterite (e.g., CZT(S,Se)) photovoltaic device andbattery.

BACKGROUND OF THE INVENTION

In order to power autonomous computers and sensors, for example in thearea of the Internet of Things (IoT), batteries as an energy sourcealone are insufficient for devices deployed in remote locations and/orin use for extended time periods. Thus, energy harvesting is required,and the most ubiquitous source of energy is light. Hence, photovoltaicdevices can be integrated with batteries to provide rechargingcapabilities. While such photovoltaic device/battery couplings exist,the available solutions have notable drawbacks such as the use ofexpensive and rare or toxic materials or, in the alternative,abundant/non-toxic silicon which is not applicable to thin filmphotovoltaics.

Ultra-small, ultra-low power computers require power to be supplied atconstant voltage. Standard lithium (Li)-ion batteries operate at ˜3-4volts (V), which is too high for low power computers. Therefore,integrated circuit (IC) voltage regulation is needed which wastesenergy. Additionally, non-toxic materials for wide deployment in theenvironment are a requirement along with a spatially small footprint.

Accordingly, improved photovoltaic device and battery integrated designswould be desirable.

SUMMARY OF THE INVENTION

The present invention provides an integrated kesterite (e.g., CZT(S,Se))photovoltaic device and battery. In one aspect of the invention, amethod of forming an integrated photovoltaic device and battery isprovided. The method includes: forming a photovoltaic device having asubstrate, an electrically conductive layer on the substrate, anabsorber layer on the electrically conductive layer, a buffer layer onthe absorber layer, a transparent front contact on the buffer layer, anda metal grid on the transparent front contact; removing the substrateand the electrically conductive layer from the photovoltaic device toexpose a backside surface of the absorber layer; forming at least oneback contact on the backside surface of the absorber layer; andintegrating the photovoltaic device with a battery, wherein the batteryincludes a positive contact and a negative contact, and wherein theintegrating includes connecting i) the positive contact of the batterywith the back contact on the backside surface of the absorber layer andii) the negative contact of the battery with the metal grid on thetransparent front contact.

In another aspect of the invention, an integrated photovoltaic deviceand battery is provided. The integrated photovoltaic device and batteryincludes: a photovoltaic device having an absorber layer, a buffer layeron the absorber layer, a transparent front contact on the buffer layer,and a metal grid on the transparent front contact; at least one backcontact on a backside surface of the absorber layer; and a batteryhaving a positive contact and a negative contact, wherein i) thepositive contact of the battery is connected to the back contact on thebackside surface of the absorber layer and ii) the negative contact ofthe battery is connected to the metal grid on the transparent frontcontact.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary methodology for forming anintegrated CZT(S,Se) photovoltaic device and battery according to anembodiment of the present invention;

FIG. 2 is a cross-sectional diagram illustrating an exemplaryconfiguration of the integrated CZT(S,Se) photovoltaic device andbattery according to an embodiment of the present invention;

FIG. 3 is a cross-sectional diagram illustrating an exemplaryconfiguration of the battery according to an embodiment of the presentinvention;

FIG. 4 is a cross-sectional diagram illustrating a substrate having alayer(s) of an electrically conductive material coated thereon accordingto an embodiment of the present invention;

FIG. 5 is a cross-sectional diagram illustrating a CZT(S,Se) absorberlayer having been formed on the conductive layer according to anembodiment of the present invention;

FIG. 6 is a cross-sectional diagram illustrating a buffer layer havingbeen formed on the CZT(S,Se) absorber layer according to an embodimentof the present invention;

FIG. 7 is a cross-sectional diagram illustrating a transparent frontcontact having been formed on the buffer layer and a metal grid havingbeen formed on the transparent front contact according to an embodimentof the present invention;

FIG. 8 is a cross-sectional diagram illustrating an isolating scribehaving been performed in the CZT(S,Se) photovoltaic device to create aplurality of isolated cells according to an embodiment of the presentinvention;

FIG. 9 is a top-down diagram illustrating contacts having been formedthat interconnect the top (negative) and bottom (positive) sides of theCZT(S,Se) photovoltaic device according to an embodiment of the presentinvention;

FIG. 10 is a cross-sectional diagram illustrating a transparent platehaving been attached to the front of the CZT(S,Se) photovoltaic deviceaccording to an embodiment of the present invention;

FIG. 11 is a cross-sectional diagram illustrating the transparent plateand CZT(S,Se) photovoltaic device having been separated from theconductive layer and substrate according to an embodiment of the presentinvention;

FIG. 12 is a cross-sectional diagram illustrating (positive) backcontacts having been formed on the (now exposed) backside surface of theCZT(S,Se) absorber layer according to an embodiment of the presentinvention;

FIG. 13 is a top-down diagram illustrating the (positive) back contactsaccording to an embodiment of the present invention;

FIG. 14 is a cross-sectional diagram illustrating an insulating filmhaving been deposited onto the positive side of the CZT(S,Se)photovoltaic device covering all but the last back contact in the seriesaccording to an embodiment of the present invention;

FIG. 15 is a cross-sectional diagram illustrating the CZT(S,Se)photovoltaic device having been integrated with the battery in astacking configuration according to an embodiment of the presentinvention;

FIG. 16 is a cross-sectional diagram illustrating an alternativeexemplary configuration of the integrated CZT(S,Se) photovoltaic deviceand battery where both the positive and negative connections occur atthe bottom of the CZT(S,Se) photovoltaic device and at the top of thebattery according to an embodiment of the present invention;

FIG. 17 is a cross-sectional diagram illustrating an alternativeexemplary configuration of the battery where both positive and negativebattery terminals are at the top of the battery according to anembodiment of the present invention; and

FIG. 18 is a cross-sectional diagram illustrating an alternativeexemplary embodiment illustrating an integrated CZT(S,Se) photovoltaicdevice and battery where a through contact is used to make contactbetween the negative battery terminal and the top/negative portion ofthe CZT(S,Se) photovoltaic device according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are non-toxic, earth abundant, inexpensive photovoltaicdevices fabricated from CZT(S,Se) operating at from about 1 volt (V) toabout 2V, and ranges therebetween, coupled with a lithium (Li)-ionbattery whose chemistry is optimized to provide voltage in this range.The present rechargeable integrated photovoltaic device and batterydesign permits operation under low light conditions (e.g., at 0.001 sunsor less), is ultra-thin (e.g., the complete integrated structure has athickness of less than about 500 micrometers (μm)), and is scalable toan area less than 100 μm.

Most commercially available photovoltaic rechargeable devices includecrystalline, polycrystalline or amorphous silicon-based solar cells.However, since silicon is a poor light absorber, the thickness of thesilicon alone must be at least about 300 micrometers (μm) and thus isnot considered “thin film” technology. Thin film photovoltaic technologyincludes absorber materials such as CZT(S,Se), CIG(S,Se), cadmiumtelluride (CdTe), and gallium arsenide (GaAs). CIG(S,Se) includes copper(Cu), indium (In), gallium (Ga), and at least one of sulfur (S) andselenium (Se). In and Ga, however, are both rare elements which makesCIG(S,Se) expensive to implement in large-scale production. CdTe usestoxic Cd, and GaAs uses toxic As and is very expensive to fabricatesince GaAs needs to be grown as a single crystal on expensive substratessuch as indium phosphide (InP).

CZT(S,Se), on the other hand, is an efficient absorber material thatincludes earth abundant, non-toxic elements. As its name implies, aCZT(S,Se) material contains copper (Cu), zinc (Zn), tin (Sn), and atleast one of sulfur (S) and selenium (Se). For a general discussion onkesterites and use of kesterite in solar cells, see, for example, Mitziet al., “Prospects and performance limitations for Cu—Zn—Sn—S—Sephotovoltaic technology,” Phil Trans R Soc A 371 (July 2013), thecontents of which are incorporated by reference as if fully set forthherein.

With regard to the battery component, typical lithium (Li)-ionchemistries have been optimized to supply from about 3V to about 4Vwhich is too high for ultra low power computers and sensors. Thus, forsuch application, the battery output needs to be stepped down in voltageusing integrated circuit (IC) voltage regulation which wastes energy.

An overview of the present techniques is now provided by way ofreference to methodology 100 of FIG. 1. In step 102, a completeCZT(S,Se)-based photovoltaic device is fabricated. By “CZT(S,Se)-based”it is meant that CZT(S,Se) serves as the absorber layer of thephotovoltaic device. Forming a complete photovoltaic device at thebeginning of the process enables all of the necessary steps to beundertaken to produce a high quality device, such as building the deviceon a molybdenum (Mo)-coated soda-lime glass (SLG) substrate (see below)which is beneficial in terms of coefficient of thermal expansion (CTE)matching between the device and the substrate, it permits the formationof beneficial secondary compounds such as MoS₂ and/or MoSe₂, etc. See,for example, Shin et al., “Control of an interfacial MoSe₂ layer inCu₂ZnSnSe₄ thin film solar cells: 8.9% power conversion efficiency witha TiN diffusion barrier,” Applied Physics Letters 101, 053903-1-4 (July2012), the contents of which are incorporated by reference as if fullyset forth herein.

In order to increase the output of the photovoltaic component undertypical indoor light conditions (e.g., 0.001 watts per square centimeter(watts/cm²) or less), it is preferable to divide the CZT(S,Se)-basedphotovoltaic device into multiple cells connected in series. Thus,according to an exemplary embodiment, the active layers of theCZT(S,Se)-based photovoltaic device (see below) are isolated via scribelines, and the individual cells connected via a metal grid to achievethe appropriate voltage.

In step 104, the complete CZT(S,Se)-based photovoltaic device is thenseparated from the substrate (e.g., from the Mo-coated SLG substrate).As will be described in detail below, separating the CZT(S,Se)-basedphotovoltaic device from the substrate can be carried out using anexfoliation process. For instance, a glass or polymer plate can beattached (e.g., using an adhesive such as an epoxy) to the top of theCZT(S,Se)-based photovoltaic device, followed by a sharp impulse on theglass plate to separate (i.e., exfoliate) the CZT(S,Se)-basedphotovoltaic device from the Mo-coated SLG substrate. See, for example,Fleutot et al., “GaSe Formation at the Cu(In,Ga)Se₂/Mo Interface-A NovelApproach for Flexible Solar Cells by Easy Mechanical Lift-Off,” Adv.Mater. Interfaces 1400044 (May 2014) (12 pages), the contents of whichare incorporated by reference as if fully set forth herein.

Separating the CZT(S,Se)-based photovoltaic device from the substrateexposes a backside surface of the CZT(S,Se) absorber. In step 106, a(positive) back contact(s) is then formed on the backside surface of theCZT(S,Se) absorber. According to an exemplary embodiment, the backcontact includes a molybdenum trioxide (MoO₃) layer on the CZT(S,Se)absorber, followed by a metal capping layer such as gold (Au). When, asdescribed above, the CZT(S,Se)-based photovoltaic device has beendivided into multiple serially connected cells, the back contact isformed individually on the backside surface of each cell. That way, theoutput from the last cell in the series can be individually obtained.

As will be described in detail below, when the CZT(S,Se)-basedphotovoltaic device is integrated with the battery component, thepositive part of the battery will be connected to the last back contactin the series with the highest voltage, while the negative part of thebattery will be connected to the tops of each cell. Thus, the top andbottom sides of the CZT(S,Se)-based photovoltaic device may also bereferred to herein as the negative and positive sides, respectively, ofthe CZT(S,Se)-based photovoltaic device. Additionally, embodiments arealso anticipated herein where both the negative and positive parts ofthe CZT(S,Se)-based photovoltaic device are accessed from the bottomside of the CZT(S,Se)-based photovoltaic device (wherein the negativepart is accessed via a through contact that passes from the top to thebottom sides of the CZT(S,Se)-based photovoltaic device).

Finally, in step 108 the CZT(S,Se)-based photovoltaic device isintegrated with the battery. As will be described in detail below,several different configurations are anticipated. For instance, in oneexemplary embodiment, the CZT(S,Se)-based photovoltaic device is placedon top of the battery where the positive and negative battery contactsare on the top and bottom of the battery, respectively, and the negativebattery contact is connected to the top side of the CZT(S,Se)-basedphotovoltaic device via a wire. In an alternative embodiment, theCZT(S,Se)-based photovoltaic device is also placed on top of thebattery, however both the positive and negative battery contacts are onthe top of the battery, and the negative battery contact is accessed atthe bottom of the CZT(S,Se)-based photovoltaic device via a throughcontact that passes from the top to the bottom sides of theCZT(S,Se)-based photovoltaic device.

An exemplary configuration of the integrated CZT(S,Se) photovoltaicdevice and battery is shown in FIG. 2. As shown in FIG. 2, the CZT(S,Se)photovoltaic device and battery are oriented in a stack. Forillustrative purposes, a low power device is shown also integrated inthe stack which runs on the power supplied by the battery. The battery,in turn, is recharged via the CZT(S,Se) photovoltaic device. Forinstance, the battery is connected to the output terminals of theCZT(S,Se) photovoltaic device that under light produces from about 1V toabout 2V, and ranges therebetween, e.g., about 1.3V, and is used tocharge the battery.

By way of example only, the Internet of Things (IoT) includes a varietyof interconnected physical entities (such as vehicles, buildings, etc.)having associated electronics such as computing devices, sensors,actuators, and communications capabilities that enable data collectionand exchange amongst the entities. Any of these IoT components can bepowered via the present CZT(S,Se) photovoltaic device/battery design.

As shown in FIG. 2, the CZT(S,Se) photovoltaic device is present on topof the battery, such that a positive part of the battery is connected toa back contact on the bottom/positive (+V) side of the CZT(S,Se)photovoltaic device, and a negative part of the battery is connected toa top/negative (−V) side of the CZT(S,Se) photovoltaic device.

The glass or polymer plate is shown attached to the top of the CZT(S,Se)photovoltaic device. As provided above, this plate is used in theexfoliation process to remove the CZT(S,Se) photovoltaic device from thesubstrate on which it is fabricated. According to an exemplaryembodiment, the glass or polymer plate has a thickness a of from about50 micrometers (μm) to about 2 mm, and ranges therebetween. Bycomparison, the CZT(S,Se) photovoltaic device and battery have acombined thickness b of from about 4 μm to about 5 μm, and rangestherebetween.

An exemplary configuration of the battery is shown in FIG. 3. As shownin FIG. 3, the battery includes a first (negative) contact (contact 1),an anode separated from a cathode by an electrolyte, and a second(positive) contact (contact 2). According to an exemplary embodiment,the battery is a Li-ion battery, wherein the cathode includes lithiumcobalt oxide (LiCoO₂), lithium iron phosphate (LiFePO₄), lithiummanganese oxide (Li₂MnO₃), and/or lithium nickel manganese cobalt oxide(LiNiMnCoO₂). According to an exemplary embodiment, a solid electrolyteis employed, such as amorphous lithium lanthanum titanate (LiLaTiO₃).The electrolyte acts as an electron barrier but allows Li ions todiffuse through the electrolyte as the battery is charged or discharged.

Suitable anode materials include, but are not limited to, vanadium(V)oxide (V₂O₅), graphite, carbon nanotubes, carbon nanofibers, silicon(Si), germanium (Ge), tin (Sn), nickel (Ni), and/or transition metaloxides such as lithium oxide and titanium oxide, etc. See, for example,Goriparti et al., “Review on recent progress of nanostructured anodematerials for Li-ion batteries,” Journal of Power Sources,” 257, pgs.421-443 (January 2014), the contents of which are incorporated byreference as if fully set forth herein. Suitable materials for the first(negative) contact include, but are not limited to, carbon (e.g., aconductive carbon paint), indium (In), etc. Suitable materials for thesecond (positive) contact include, but are not limited to, nickel (Ni)and/or copper (Cu). As shown in FIG. 3, the battery less the contacts(i.e., the anode, cathode, and electrolyte) has a combined thickness cof from about 1.5 μm to about 3 μm, and ranges therebetween.

Given the above overview of the present techniques, exemplaryembodiments for forming the present integrated CZT(S,Se) photovoltaicdevice and battery are now described by way of reference to FIGS. 4-18.The embodiments each follow the same general process flow outlined inFIG. 1. Thus, reference will be made in the following description to theassociated steps of methodology 100. For instance, in step 102 ofmethodology 100 a completed CZT(S,Se) device is fabricated.

According to an exemplary embodiment, the CZT(S,Se)-based photovoltaicdevice fabrication process begins with a suitable device substrate 402coated with a layer 404 (or optionally multiple layers representedgenerically by layer 404) of an electrically conductive material. SeeFIG. 4.

Suitable substrates include, but are not limited to, soda lime glass(SLG), ceramic, metal foil, or plastic substrates. Suitable materialsfor forming conductive layer 404 include, but are not limited to,molybdenum (Mo), molybdenum trioxide (MoO₃), gold (Au), nickel (Ni),tantalum (Ta), tungsten (W), aluminum (Al), platinum (Pt), titaniumnitride (TiN), silicon nitride (SiN), and combinations thereof.According to an exemplary embodiment, the conductive layer 404 is coatedon the substrate 402 to a thickness of from about 100 nanometers (nm) toabout 500 nm, and ranges therebetween. In general, the various layers ofthe device will be deposited sequentially using a combination ofvacuum-based and/or solution-based approaches. For example, theelectrically conductive material 404 can be deposited onto the substrate402 using evaporation or sputtering.

Next, a CZT(S,Se) absorber layer 502 is formed on the conductive layer404. See FIG. 5. As highlighted above, the CZT(S,Se) absorber layer 502contains Cu, Zn, Sn, and at least one of S and Se. According to anexemplary embodiment, the CZT(S,Se) absorber layer 502 is formed havinga total thickness of from about 0.5 μm to about 2 μm, and rangestherebetween.

The CZT(S,Se) absorber layer 502 can be formed using vacuum-based,solution-based, or other suitable approaches to form a stack of layers.See for example U.S. Pat. No. 8,426,241 by Ahmed et al., entitled“Structure and Method of Fabricating a CZTS Photovoltaic Device byElectrodeposition,” the contents of which are incorporated by referenceas if fully set forth herein. The sequence of the layers in the stackcan be configured to achieve optimal band grading and/or adhesion to thesubstrate. See, for example, Dullweber et al., “Back surface band gapgradings in Cu(In,Ga)Se₂ solar cells,” Thin Solid Films, vol. 387, 11-13(May 2001), the contents of which are incorporated by reference as iffully set forth herein.

Suitable solution-based kesterite fabrication techniques are described,for example, in U.S. Patent Application Publication Number 2013/0037111by Mitzi et al., entitled “Process for Preparation of ElementalChalcogen Solutions and Method of Employing Said Solutions inPreparation of Kesterite Films,” the contents of which are incorporatedby reference as if fully set forth herein. Suitable particle-basedprecursor approaches for CZT(S,Se) formation are described, for example,in U.S. Patent Application Publication Number 2013/0037110 by Mitzi etal., entitled “Particle-Based Precursor Formation Method andPhotovoltaic Device Thereof,” the contents of which are incorporated byreference as if fully set forth herein.

As will be described in detail below, the CZT(S,Se) photovoltaic deviceis preferably divided into a plurality of serially connected cells.According to an exemplary embodiment, the CZT(S,Se) absorber layer 502is configured to reach higher single cell operating voltages(Vmpp=maximum power point voltage) by replacing some of the Se in thelattice with S which increases the band gap and hence the voltage. Forinstance, under 1 sun illumination open circuit voltages (Voc) ofindividual cells is preferably up to about 680 millivolts (mV). This isa distinct advantage for CZT(S,Se) because the band gap can be modifiedby the ratio of S to Se in the material.

For instance, during formation of the CZT(S,Se) absorber layer 502 the Sand Se can be introduced via separately regulated (valve controlled)sources to control the ratio of S to Se. See, for example, U.S. PatentApplication Publication Number 2012/0100663 filed by Bojarczuk et al.,entitled “Fabrication of CuZnSn(S,Se) Thin Film Solar Cell with ValveControlled S and Se,” the contents of which are incorporated byreference as if fully set forth herein. For instance, pure sulfur(sulfide) would give the CZT(S,Se) absorber layer 502 a band gap ofabout 1.5 electron volts (eV) whereas pure selenium (selenide) wouldgive the CZT(S,Se) absorber layer 502 a band gap of about 0.96 eV.According to an exemplary embodiment, the final CZT(S,Se) absorber layer502 (i.e., post formation and anneal) has an S:Se ratio of from about 0(i.e., pure Se) to about 1 (pure S), and ranges therebetween, e.g., fromabout 0.05 to about 0.95, and ranges therebetween.

Since the as-deposited materials have poor grain structure and a lot ofdefects, following deposition of the CZT(S,Se) materials a post annealin a chalcogen environment is preferably performed. An anneal in achalcogen (e.g., S and/or Se) environment improves the grain structureand defect landscape in the CZT(S,Se) material. According to anexemplary embodiment, the anneal is performed at a temperature of fromabout 500 degrees Celsius (° C.) to about 600° C., and rangestherebetween, to form a film composed of polycrystalline grains of from1 μm to about 2 μm, and ranges therebetween, in size. Grain size ismeasured, for example, as the greatest length of the grain when viewedin cross-section. Performing the anneal in a chalcogen environmentprovides another opportunity to tune the S:Se ratio. Specifically, beingvolatile species, S and Se will evaporate from the film during theanneal. The chalcogen environment serves to replace these volatiles.Thus, by regulating a S:Se ratio in the chalcogen environment during theanneal, one can control the final S:Se ratio in the film. As providedabove, the final CZT(S,Se) absorber layer 502 (i.e., post formation andanneal) is targeted to have an S:Se ratio of from about 0 (i.e., pureSe) to about 1 (pure S), and ranges therebetween, e.g., from about 0.05to about 0.95, and ranges therebetween.

A thin film buffer layer 602 is then formed on the CZT(S,Se) absorberlayer 502. See FIG. 6. The buffer layer 602 forms a p-n junction withthe CZT(S,Se) absorber layer 502. According to an exemplary embodiment,the buffer layer 602 is formed having a thickness of from about 100angstroms (Å) to about 1,000 Å, and ranges therebetween.

Suitable buffer layer materials include, but are not limited to, cadmiumsulfide (CdS), a cadmium-zinc-sulfur material of the formulaCd_(1-x)Zn_(x)S (wherein 0<x≤1), indium sulfide (In₂S₃), zinc oxide,zinc oxysulfide (e.g., a Zn(O,S) or Zn(O,S,OH) material), and/oraluminum oxide (Al₂O₃). According to an exemplary embodiment, the bufferlayer 602 is deposited on the CZT(S,Se) absorber layer 502 usingstandard chemical bath deposition.

A transparent front contact 702 is then formed on the buffer layer 602.See FIG. 7. Suitable transparent front contact materials include, butare not limited to, transparent conductive oxides (TCOs) such asindium-tin-oxide (ITO) and/or aluminum (Al)-doped zinc oxide (ZnO)(AZO)). According to an exemplary embodiment, the transparent frontcontact 702 is formed on the buffer layer 602 by sputtering.

A metal grid 704 is than formed on the transparent front contact 702.Suitable materials for forming the metal grid 704 include, but are notlimited to, Ni and/or Al. According to an exemplary embodiment, themetal grid 704 is formed on the transparent front contact 702 usingevaporation or sputtering. According to an exemplary embodiment, aportion of the metal grid 704 is a through contact that passes from atop to a bottom of the CZT(S,Se) photovoltaic device. See, for example,FIG. 7 where it is shown that the metal grid 704 can pass through thedevice stack. It is notable that while dotted lines are used inconjunction with one of the contacts 704 to show that it passes throughthe device stack, in practice all contacts 704 shown would pass throughthe stack. Light shines down on the device from the front where themetal grid 704 is used to collect the electrons in the front (i.e.,negative contact). This negative charge needs to be conveyed to the backwhich is done via the through contacts. See also FIG. 13. This throughcontact configuration will allow access to the top/negative part of theCZT(S,Se) photovoltaic device from the bottom side simply by accessingthe through contact at the bottom of the CZT(S,Se) photovoltaic device.As such, battery/CZT(S,Se) photovoltaic device configurations can beimplemented where both positive and negative terminals of the batteryare on top of the battery, and are integrated with the positive andnegative parts of the CZT(S,Se) photovoltaic device when the CZT(S,Se)photovoltaic device is placed on top of the battery. See below. As alsoshown in FIG. 7, metal lines 706 may be formed along the top (negative)side of the CZT(S,Se) photovoltaic device.

As provided above, it is desirable to create a plurality of individualserially-connected cells. To do so, an isolating scribe is nextperformed to divide the CZT(S,Se) photovoltaic device into multiplecells. See FIG. 8. As shown in FIG. 8, scribe lines are formed in theCZT(S,Se) photovoltaic device (i.e., extending through the metal lines706, the transparent front contact 702, buffer layer 602, and CZT(S,Se)absorber layer 502) down to the conductive layer 404. As a result, aplurality of isolated cells is formed. It is notable that the conductivelayer 404 and substrate 402 will be removed, and replaced withindividual positive back contacts. Thus, each cell will eventually beelectrically isolated. It is also notable that since the scribe linesare needed to isolate individual cells they need to be done at somepoint in the process, but not necessarily at the specific pointillustrated in FIG. 8. For instance, it is possible to perform theisolating scribe earlier in the process.

FIG. 9 is a top-down view of the negative side of the CZT(S,Se)photovoltaic device (for example, from vantage point A, see FIG. 8).FIG. 9 provides a top-down view of the metal grid 704. As shown in FIG.9, the metal grid 704 includes a ladder-like structure at the top of theCZT(S,Se) photovoltaic device that assists in collection of electrons.Also, as described above, a portion of the metal grid 704 can beconfigured as a through contact that passes from the top to the bottomof the device stack and thus enables access to the top/negative part ofthe CZT(S,Se) photovoltaic device from the bottom of the stack. See FIG.9. To form the through contact portion of metal grid 704, vias may bepatterned in the CZT(S,Se) photovoltaic device (down to the conductivelayer 404) and then filled with a metal, Cu, Al, and/or Ni. The metallines 706 can be formed by selectively depositing one or more of thesemetals using a process such as evaporation (e.g., through a shadowmask).

Switching back to a cross-sectional view, a transparent plate 1002 isnext attached to the front of the CZT(S,Se) photovoltaic device. SeeFIG. 10. According to an exemplary embodiment, plate 1002 includes atransparent glass or polymer plate that is attached to the front of theCZT(S,Se) photovoltaic device using an adhesive such as a clear epoxy.The transparent plate 1002 secures the individual cells, and enables theremoval of the underlying conductive layer 404 and substrate 402 (as perstep 104 of methodology 100—see above).

Specifically, as shown in FIG. 11 the transparent plate 1002 andCZT(S,Se) photovoltaic device (i.e., transparent front contact 702,buffer layer 602, and CZT(S,Se) absorber layer 502) are separated fromthe conductive layer 404 and substrate 402. According to an exemplaryembodiment, an exfoliation process is employed whereby a sharp impulseon the transparent plate 1002 is used to cleave the CZT(S,Se)photovoltaic device from the conductive layer 404/substrate 402. By wayof example only, a square cross-section rod can be placed against thetransparent plate 1002 and struck with a hammer. This sideways ‘shear’impulse separates the plate+device from the conductive layer+substrate.

The backside surface of the CZT(S,Se) absorber layer 502 (i.e., a sideof the CZT(S,Se) absorber layer 502 opposite the buffer layer 602) isnow exposed. The conductive layer 404/substrate 402 can be discarded orused in the production of additional CZT(S,Se) photovoltaic devices.

As per step 106 of methodology 100, at least one (positive) back contact1202 is next formed on the (now exposed) backside surface of theCZT(S,Se) absorber layer 502. See FIG. 12. In the example shown in FIG.12, one back contact 1202 is formed for each cell. Thus, each individualcell will have its own back contact 1202. According to an exemplaryembodiment, the back contact(s) 1202 is/are formed by first depositing aconductive material 1202 a, followed by a capping layer 1202 b. See FIG.12. According to an exemplary embodiment, conductive material 1202 a isMoO₃, MoO₂, alumina (Al₂O₃), titanium dioxide (TiO₂), and/or selenium(Se), and the capping layer 1202 b is Au. In FIG. 12 the devicestructure is shown flipped such that the backside surface of theCZT(S,Se) absorber layer 502 is now on top. This flipping is done forease of manufacture such that the additional device layers (i.e.,conductive material 1202 a, capping layer 1202 b, etc.) can be formedsequentially, one on top of the other.

According to one exemplary embodiment, the back contact corresponding tothe last cell in the series is thicker than the others since it is thatlast back contact that will interface with the battery. As will bedescribed below, the other contacts will be covered with an insulatingfilm to isolate them from the battery. Alternatively, another backcontact can also be implemented to make (bottom-side) contact to thenegative battery terminal (via the through contact—see above). Thisexemplary embodiment will be described in detail below. In that case, itis desirable to have thicker back contacts at both the beginning(negative −V contact) and at the end (positive +V contact) of theseries. See below.

Switching briefly again to a top-down view (this time of the positiveside of the CZT(S,Se) device), FIG. 13 depicts (from a vantage point B,see FIG. 12) the (positive) back contacts 1202. As shown in FIG. 13, thethrough contacts 704 pass from the top (negative) to the bottom(positive—as depicted in FIG. 13) sides of the CZT(S,Se) device. As alsoshown in FIG. 13, the through contact 704 of one cell is wired to theback contact 1202 of the previous, adjacent cell in the series. Thatway, as will be described in detail below, the (positive) region of thebattery can be simply connected to the last cell in the series.

Next, as shown in FIG. 14, an insulating film 1402 is deposited onto thepositive side of the CZT(S,Se) device over all but the last back contact1202 in the series. The insulating film 1402 will isolate the individualback contacts 1202 from the positive region of the battery save for thelast contact with the highest voltage needed to charge the battery.Suitable insulators include, but are not limited to, semiconductorinsulators such as an oxide material, plastic, glass, etc.

With the insulating film in place, the CZT(S,Se) photovoltaic device canthen be integrated with the battery as per step 108 of methodology 100.See FIG. 15. The CZT(S,Se) photovoltaic device is shown flipped in FIG.15 such that the back contacts 1202 are facing the positive (+V) batterycontact. The CZT(S,Se) photovoltaic device is then placed on top of thebattery such that the last back contact 1202 physically contacts thepositive battery contact. According to an exemplary embodiment, theCZT(S,Se) photovoltaic device is secured to the battery using anadhesive (such as an epoxy) between the insulating film 1402 and thepositive side of the battery.

The insulating film 1402 separates all but the last back contact 1202from the positive battery contact. Thus, only the last cell in theseries is connected to the positive side of the battery. A negativecontact is created through a wire (as shown) or via from the metal grid704 on the front/negative (−V) side of the CZT(S,Se) photovoltaic deviceto the negative contact on the battery.

In order to simplify the design, and thereby eliminate the need for anadded wire/via to connect from the negative battery terminal to thetop/negative side of the CZT(S,Se) photovoltaic device, an alternativeembodiment is now presented where the through contact portions of themetal grid 704 are accessed in conjunction with a battery design havingboth positive and negative terminals on the top of the battery. The samegeneral fabrication process is employed and thus, the like structuresare numbered alike in the following description.

Referring to FIG. 16, an alternative exemplary configuration of theintegrated CZT(S,Se) photovoltaic device and battery is shown. As shownin FIG. 16, the CZT(S,Se) photovoltaic device and battery are orientedin a stack. For illustrative purposes, a low power device is shown alsointegrated in the stack which runs on the power supplied by the battery.Exemplary low power devices (such as IoT devices) that can be poweredvia this CZT(S,Se) photovoltaic device/battery design were described indetail above. In this example, both positive and negative parts of thebattery are present on top of the battery. The CZT(S,Se) photovoltaicdevice is present on top of the battery such that a positive part of thebattery is connected to a back contact on the bottom/positive (+V) sideof the CZT(S,Se) photovoltaic device, and a positive part of the batteryis connected to another back contact on the bottom/negative (−V) side ofthe CZT(S,Se) photovoltaic device.

The glass or polymer plate is shown attached to the top of the CZT(S,Se)photovoltaic device. As provided above, this plate is used in theexfoliation process to remove the CZT(S,Se) photovoltaic device from thesubstrate on which it is fabricated. According to an exemplaryembodiment, the glass or polymer plate has a thickness (in this casedimension d) of from about 50 μm to about 2 mm, and ranges therebetween.The CZT(S,Se) photovoltaic device has a thickness e of from about 1 μmto about 5 μm, and ranges therebetween, the battery has a thickness f offrom about 400 μm to about 1 mm, and ranges therebetween, and theCZT(S,Se) photovoltaic device and battery are separated by a distance gof from about 100 nanometers (nm) to about 1 mm, and rangestherebetween.

An exemplary configuration of the battery is shown in FIG. 17. As shownin FIG. 17, in the same manner as described above, the battery includesa first (negative) contact (contact 1), an anode separated from acathode by an electrolyte, and a second (positive) contact (contact 2).According to an exemplary embodiment, the battery is a Li-ion battery.Suitable anode, cathode, and electrolyte materials were provided above.

In this example however access to positive and negative contacts ispresent at the top of the battery. Specifically, a leg of the bottomcontact in the battery stack (the negative battery contact in thisexample) extends (off to one side) up to the top of the stack, and isseparated from the layers it passes via an electrical insulator.

Thus, when the CZT(S,Se) photovoltaic device is placed on top of thebattery, both positive and negative connections occur at the top of thebattery/bottom of the CZT(S,Se) photovoltaic device. See FIG. 18. Asshown in FIG. 18, the through contact portion of metal grid 704 (shownin dotted lines to illustrate how the through contact passes from thetop of the stack through to the bottom of the stack) makes theconnection from the negative battery terminal to the negative/topportion of the CZT(S,Se) photovoltaic device.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. An integrated photovoltaic device and battery,comprising: a photovoltaic device having an absorber layer, a bufferlayer on the absorber layer, a transparent front contact on the bufferlayer, and a metal grid on the transparent front contact; at least oneback contact on a backside surface of the absorber layer; and a batteryhaving a positive contact and a negative contact, wherein i) thepositive contact of the battery is connected to the back contact on thebackside surface of the absorber layer and ii) the negative contact ofthe battery is connected to the metal grid on the transparent frontcontact.
 2. The integrated photovoltaic device and battery of claim 1,wherein the absorber layer comprises a kesterite material.
 3. Theintegrated photovoltaic device and battery of claim 2, wherein theabsorber layer comprises copper, zinc, tin, and at least one of sulfurand selenium.
 4. The integrated photovoltaic device and battery of claim3, wherein the absorber layer has a sulfur to selenium ratio of fromabout 0.05 to about 0.95, and ranges therebetween.
 5. The integratedphotovoltaic device and battery of claim 1, wherein the batterycomprises a lithium ion battery.
 6. The integrated photovoltaic deviceand battery of claim 1, wherein the battery comprises: a cathodeadjacent to the positive contact; an anode adjacent to negative contact;and a solid electrolyte separating the cathode and the anode.
 7. Theintegrated photovoltaic device and battery of claim 6, wherein thecathode comprises a material selected from the group consisting of:lithium cobalt oxide, lithium iron phosphate, lithium manganese oxide,lithium nickel manganese cobalt oxide, and combinations thereof.
 8. Theintegrated photovoltaic device and battery of claim 6, wherein the anodecomprises a material selected from the group consisting of: vanadium(V)oxide, graphite, carbon nanotubes, carbon nanofibers, silicon,germanium, tin, nickel, lithium oxide, titanium oxide, and combinationsthereof.
 9. The integrated photovoltaic device and battery of claim 6,wherein the solid electrolyte comprises amorphous lithium lanthanumtitanate.
 10. The integrated photovoltaic device and battery of claim 1,wherein the photovoltaic device is divided into multiple individualcells.
 11. The integrated photovoltaic device and battery of claim 10,wherein the multiple individual cells are connected in series.
 12. Theintegrated photovoltaic device and battery of claim 1, wherein thephotovoltaic device is present on top of the battery.
 13. The integratedphotovoltaic device and battery of claim 12, wherein the positivecontact of the photovoltaic device in physical contact with the positivecontact of the battery.
 14. The integrated photovoltaic device andbattery of claim 1, further comprising: a wire connecting the negativecontact of the battery to the metal grid on the transparent frontcontact.
 15. The integrated photovoltaic device and battery of claim 1,further comprising: at least one first back contact and at least onesecond back contact on the backside surface of the absorber layer. 16.The integrated photovoltaic device and battery of claim 15, wherein thepositive contact of the battery is connected to the first back contact,and the negative contact of the battery is connected to the second backcontact.
 17. An integrated photovoltaic device and battery, comprising:a photovoltaic device having an absorber layer, a buffer layer on theabsorber layer, a transparent front contact on the buffer layer, and ametal grid on the transparent front contact, wherein the absorber layercomprises a kesterite material comprising copper, zinc, tin, and atleast one of sulfur and selenium; at least one back contact on abackside surface of the absorber layer; and a battery having a positivecontact and a negative contact, wherein i) the positive contact of thebattery is connected to the back contact on the backside surface of theabsorber layer and ii) the negative contact of the battery is connectedto the metal grid on the transparent front contact, wherein the batteryfurther comprises a cathode adjacent to the positive contact, an anodeadjacent to negative contact, and a solid electrolyte separating thecathode and the anode.
 18. The integrated photovoltaic device andbattery of claim 17, wherein the cathode comprises a material selectedfrom the group consisting of: lithium cobalt oxide, lithium ironphosphate, lithium manganese oxide, lithium nickel manganese cobaltoxide, and combinations thereof.
 19. The integrated photovoltaic deviceand battery of claim 17, wherein the anode comprises a material selectedfrom the group consisting of: vanadium(V) oxide, graphite, carbonnanotubes, carbon nanofibers, silicon, germanium, tin, nickel, lithiumoxide, titanium oxide, and combinations thereof.
 20. The integratedphotovoltaic device and battery of claim 17, wherein the solidelectrolyte comprises amorphous lithium lanthanum titanate.